The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to utilize the bandwidth of existing fiber-optic cable more efficiently. An established method for increasing the carrying capacity of existing fiber cable is Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.”
In this specification, the individual information-carrying lights within a WDM optical communications system are referred to as either “signals” or “channels.” The term “channel” is also utilized herein to refer to a continuous-wave (CW) light within a restricted wavelength band that is carried by an optical fiber. Such a CW light is not modulated and does not carry information (other than the information of its own existence) and therefore cannot be properly referred to as a “signal”. In general, however, modulation may be imposed upon a CW light carried by an optical fiber communications system. This modulation imparts information to the light and will, in general, be performed at some location remote from the original source of the light. At this point, the light becomes a “signal” and is no longer a “CW” light. The new signal resides within the same wavelength band as the original CW light from which it was formed. Downstream modulation of CW light sources in this fashion is a characteristic of an optical communications system comprising centralized light sources.
The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.” The totality of channels carried by an optical fiber, optical line or optical system, without regard to whether the channels comprise signals or CW lights, is referred to herein as a “composite optical transmission”. The term “wavelength,” denoted by the Greek letter λ (lambda) is used synonymously with the terms “signal” or “channel” and may refer to either a signal-carrying light or a CW light. Although each information carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, λ, and a plurality of n such channels are referred to as “n wavelengths” denoted λ1-λn.
In most communications systems, it is desirable to provide for full-duplex communications—that is, simultaneous information transfer from a first point to a second point and from the second point back to the first point. In conventional fiber optic communications systems, such full-duplex communication is accomplished by providing both a light source (“transmitter”) and a light detector (“receiver”) at both the first point and the second point. Information to be transferred from, for instance, a central office to customer or subscriber premises, is converted from electrical format to optical format and the optical formatted signal is transferred along a fiber optic communications system to the customer or subscriber premises. The receiver at the customer or subscriber premises converts the optical formatted information back to electrical format for conveyance to its final destination (a data reception device such as a computer). Any return or response information generated at the customer premises is generated as an electrical signal, transformed to an optical signal by a separate transmitter at the customer premises, and transferred back to a central office over the fiber optic communications system.
Historically, the conventional communications system described above has served its purposes well. However, this conventional system requires light sources at each customer or subscriber location. The provision of and maintenance of a separate light source at each such remote location is costly and wasteful of resources, especially as the number of channels and total data traffic increases within the communications system.
Accordingly, there exists a need for an improved full-duplex optical communications system. The system should not require separate light sources at customer or subscriber premises. The present invention addresses such a need.